The invention relates to a method and an apparatus according to the preambles of independent claims 1 and 5.
High-power frequency converters provided with a voltage intermediate circuit conventionally comprise a rectifier, which is separate from the inverter part and implemented as an n-phased half-controlled diode bridge. A capacitor battery of a DC intermediate circuit is fed by the rectifier to produce direct voltage in the intermediate circuit. The lower branches of such a rectifier bridge are diodes, and the upper branches are controllable components, most typically thyristors.
When the rectifier is switched on, it must be ensured before the thyristors are controlled into the diode mode that the capacitor battery of the intermediate circuit has been charged nearly up to its final voltage, because otherwise a high current pulse taken from the network may burn protective fuses. The charging may be implemented by using either a resistor or adjusting the phase angle of the thyristors. The purpose is to charge the capacitor battery up to its maximum voltage by restricting the magnitude of the current flowing into the capacitor of the intermediate circuit. The components in the upper branch cannot be controlled with full control until the voltage of the intermediate circuit is at its maximum or close to it, in which case the components functionally correspond to diodes, i.e. they are conductive always when this is enabled by the voltage acting over the component. If the components in the upper branch are implemented as diodes, current has to be restricted resistively.
U.S. Pat. No. 6,038,155 discloses an integrated circuit intended to control the phase angles of the thyristors in the upper branch of the rectifier for charging the capacitor battery.
A problem associated with prior art is that the rectifier control is bound to a certain voltage level which is to be reached in the DC voltage circuit. In that case, the rectifier bridge has to be adapted for each line voltage and information on the voltage to be reached has to be provided for the rectifier bridge controller. In addition, in several solutions, the size of the capacitor battery of the intermediate circuit, the frequency of the supply network and the order and number of phases have to be taken into account in the rectifier bridge control. This limits the versatility of the controller and its adaptability to changing conditions since the changes have to be always accounted for in the commissioning stage.
Furthermore, prior art devices often suffer from problems caused by ground faults in the inverter output and intermediate circuit and by general interference in the supply network. The rectifier part of the frequency converter should be able to feed direct voltage into the capacitor battery and further into the inverter part to ensure reliable operation regardless of frequency changes or voltage interferences in the supply network. In the worst case, a momentary interference in the supply voltage, such as a drop in the frequency or voltage level, results in a decrease in the voltage of the intermediate circuit in the frequency converter. When the interference disappears, current is still fed into the capacitor battery at full control regardless of the decrease in the voltage of the capacitor battery. This results in a considerable overcurrent, which burns the overcurrent protectors, and consequently the use is interrupted.